Description

Schistosomiasis (also known as bilharzia) is a parasitic infection caused by Schistosoma flukes, which have complex life cycles involving specific freshwater snail species as intermediate hosts. Infected snails release large numbers of minute, free-swimming larvae (cercariae) that are capable of penetrating the unbroken skin of the human host. Even brief exposure to contaminated freshwater (e.g., during wading, swimming, or bathing) can result in infection. Human schistosomiasis cannot be acquired by contact with salt water (oceans or seas). However, the cercariae of birds and aquatic mammals can penetrate the skin of human beings who enter infested fresh or salt water in many parts of the world, including cool temperate areas. These cercariae die in the skin but may elicit a pruritic rash (“swimmer’s itch” or “clam-digger’s itch”). Organisms causing hepatic or intestinal schistosomiasis include Schistosoma mansoni, S. mekongi, S. intercalatum, and S. malayanensis. S. haematobium causes urinary schistosomiasis (1,2).

Occurrence

This infection occurs widely throughout the tropics and subtropics, affecting some 200 million persons (see Map 4-11). Schistosomiasis is most prevalent in sub-Saharan Africa. In highly disease-endemic areas, prevalence rates can exceed 50% among the local population, and high rates have been reported among expatriates living in such areas and even among short-term travelers to these areas (1-3).

Risk for Travelers

Exposure to schistosomiasis is a health hazard for persons who travel to disease-endemic areas. Outbreaks of schistosomiasis have occurred among adventure travelers on river trips in Africa, as well as among resident expatriates, such as Peace Corps volunteers in high-risk areas (4,5). Those at greatest risk are travelers who wade, swim, or bathe in freshwater in areas where sanitation is poor and the snail hosts are present.

Clinical Presentation

Clinical manifestations of acute infection can occur within 2-12 weeks of exposure to cercariae-infested water, but most acute infections are asymptomatic. The most common acute syndrome is Katayama fever. Symptoms include fever, loss of appetite, weight loss, abdominal pain, hematuria, weakness, headaches, joint and muscle pain, diarrhea, nausea, and cough. Rarely, the central nervous system can be involved, producing seizures or transverse myelitis as a result of mass lesions of the brain or spinal cord. Chronic infections can cause disease in the liver, intestinal tract, bladder (including bladder cancer), kidneys, or lung. Many persons with chronic infections recall no symptoms of acute infection. Diagnosis of infection is usually confirmed by serologic studies or by finding schistosome eggs on microscopic examination of stool or urine. Schistosome eggs can be found as soon as 6-8 weeks after exposure, but are not always detectable. Serologic testing in the exposed, yet asymptomatic traveler should ideally be performed several months following exposure (6-8).

Prevention

No vaccine is available, nor are any drugs recommended as chemoprophylactic agents at this time. Because there is no practical way for the traveler to distinguish infested from noninfested water, travelers should be advised to avoid wading, swimming or other contact with freshwater in disease-endemic countries. Untreated piped water coming directly from canals, lakes, rivers, streams or springs may contain cercariae, but heating bathing water to 50° C (122° F) for 5 minutes or filtering water with fine-mesh filters can eliminate the risk of infection. If such measures are not feasible, travelers should be advised to allow bathing water to stand for 2 days, because cercariae rarely remain infective longer than 24 hours. Swimming in adequately chlorinated swimming pools is virtually always safe, even in disease-endemic countries. Vigorous towel-drying after accidental exposure to water has been suggested as a way to remove cercariae before they can penetrate the skin; however, this may prevent only some infections and should not be recommended to travelers as a preventive measure. Although topical application of the insect repellent DEET can block penetrating cercariae, the effect is short lived and cannot reliably prevent infection (6).

Upon return from foreign travel, persons who may have been exposed to schistosome-infested freshwater should be advised to undergo screening tests. Because serologic tests are more sensitive than microscopic examination of stool and urine for eggs, previously uninfected but potentially exposed travelers should be tested for antibodies to schistosomes if microscopic examination of stool and urine for eggs is negative or not available. CDC performs a screening ELISA that is 99%, 90%, and 50% sensitive for Schistosoma mansoni, S. haematobium, and S. japonicum, respectively, and a confirmatory, species-specific immunoblot that is at least 95% sensitive and 99% specific for all three species (9). Serologic tests performed in commercial laboratories may not be as sensitive or specific.

Treatment

Safe and effective oral drugs are available for the treatment of schistosomiasis. Praziquantel is the drug of choice for all species of Schistosoma (10). Travelers who suspect they may have schistosomiasis should be advised to contact an infectious disease or tropical medicine specialist.

References

1. World Health Organization. The control of schistosomiasis. Second report of the WHO Expert Committee. World Health Organ Tech Rep Ser. 1993;830:1-86.
2. WHO Expert Committee. Prevention and control of schistosomiasis and soil-transmitted helminthiasis. World Health Organ Tech Rep Ser. 2002;912:1-57.  
3. Savioli L, Albonico M, Engels D, Montresor A. Progress in the prevention and control of schistosomiasis and soil-transmitted helminthiasis. Parasitol Int. 2004;53:103-13.  
4. CDC. Schistosomiasis in U.S. Peace Corps volunteers—Malawi, 1992. MMWR Morbid Mortal Wkly Rep. 1993;42:565-70.
5. Cetron MS, Chitsulo L, Sullivan JJ, Pilcher J, Wilson M, Noh J, et al. Schistosomiasis in Lake Malawi. Lancet. 1996;348:1274-8.
6. Corachan M. Schistosomiasis and international travel. Clin Infect Dis. 2002;35:446-50.  
7. Jordan P, Webbe G, Sturrock RF, eds. Human schistosomiasis. Wallingford: CAB International; 1993.  
8. Ross AG, Bartley PG, Sleigh AC, Olds GR, Li Y, Williams GM, et al. Schistosomiasis. N Engl J Med. 2002;346:1212-20.  
9. Tsang VC, Wilkins PP. Immunodiagnosis of schistosomiasis. Screen with FAST-ELISA and confirm with immunoblot. Clin Lab Med. 1991;11:1029-39.
10. Utzinger J, Keiser J. Schistosomiasis and soil-transmitted helminthiasis: common drugs for treatment and control. Expert Opin Pharmacother. 2004;5:263-285.

MAP 4-11 Geographic distribution of schistosomiasis.

Note that the prevalence of schistosomiasis is changing rapidly. Control programs have eliminated or greatly reduced transmission of schistosomiasis in most countries in Asia and the Americas. On the other hand, the prevalence of schistosomiasis has increased in sub-Saharan Africa. Water resource development projects and population movements have led to introduction of schistosomiasis into regions and countries that had not been endemic for the disease. Japan and Montserrat have eliminated schistosomiasis, and the risk is believed to be extremely low or nonexistent in the shaded countries indicated on the map. In Brazil, China, Egypt, the Philippines, Iran, Morocco, Venezuela, and elsewhere, national control programs have reduced morbidity due to schistosomiasis. (Updated August 30, 2007)